needing low speed SPI communication

[abbas]

Posted on April 12, 2015 at 16:50

Hello everyone!

I am using STM32F103RET for my project. I need to communicate by SPI with a low speed digital device with the speed of at most 1KHz clock. At the same time, I need to have my own cpu clock as high as possible. As I saw at the reference manual, I just can decrease SPI clock to APB2/256 which is too high ( at the best situation, CPU_clock/16/256 -> 72MHz/16/256 = 17KHz - by loosing APB2 speed to CPU_clock/16 ) have I hopefully, missed a point? Is there any way to have lower speed SPI with keeping system speed high? #stm32f10x

[Tesla DeLorean]

Posted on April 12, 2015 at 22:57

Is there any way to have lower speed SPI with keeping system speed high?

Pretend to be a Slave on the bus, an supply the clock? I2S mode might afford more rates. Other than that perhaps consider bit-banging the bus via GPIO pins in the 1ms ticker? SPI is one of the areas where the STM32 is unduly inflexible.

[Chris1]

Posted on April 13, 2015 at 15:44

You may be able to use a USART, which can be set to low baud rates.  For example, we have used USART3 on the F103 for SPI as a master driving MOSI. 

Depending on the needs of the attached device, you may have to reorder the bits sent.

[abbas]

Posted on April 15, 2015 at 12:24

Thanks clive1

I reached to this same solution. Here, the device was a shift register which the digital lines were passed through opto-couplers ( that limit the rate ) and I was about to clock the register by mcu. By the way, it was weird to here this limitation in my favorite STM MCUs!

[abbas]

Posted on April 15, 2015 at 12:30

Thanks Chris T

Your solution looks really smart! I may test it if the latter solution were in trouble. If I knew it, I would consider it in my PCB